Abstract: The paper discusses the RT fixed position detecting technology to backing plate welding joints of stainless steel liner which can show the detail locations to the welding-default in the joint, as well as the major detecting items. The paper also summarizes the detection key point in RT, shows us the RT second exposure contrast of the defects location and the accurate fixed position during plotting method defect, and tells how to judge the hidden depth of the defects in construct period which can shorten the criterion period. Therefore, the method can directs us how to be in good progress on the project. At the same time the paper tells us the RT method in three-dimensional RT fixed position detecting technology, which can accurately fix position and judge the hidden depth of the defects. The technology can be used in the steel structure, pressure vessel, the soldering seal seam to boiler defect, and can decrease unnecessary repair to welding joint, hence reducing the production cycle.